Gram-negative bacteria have similar LPS structures. Some mutant gram-negative bacterial strains lack the O-carbohydrate side-chains normally associated with gram-negative bacteria. These mutant organisms lack pili and outer antigens which are normally associated with the LPS membrane leaving LPS and other core antigens exposed.
Escherichia coli strain J5 is a well known example of a genetically stable gram-negative bacterial species having LPS and other core antigens exposed. Other gram-negative bacteria of different species, such as Salmonella enteritidis, ATCC No. 53000, described in European patent application No. 0158282, also lack the O-carbohydrate side-chains (also known as "K antigens"). The European patent application teaches a method of preparing non-O-carbohydrate side-chain gram-negative bacteria.